watchme
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My thoughts...
Although the Grande Duo is a "true GMT" (meaning the 2nd time-zone can go forwards or backwards through the date with the adjusting in either direction), I, personally, like the Grande GMT better.
Why?
(1) 8-day power reserve (actually 9.25 days, but JLC conservatively says 8 for accuracy).
(2) grande date.
(3) push-buttons for time differential (vs pulling the crown)
Now, for you, if you want simplicity, go with the Grande Duo. If you want mechanical brilliance, go with the Grande GMT. The latter speaks to my German, machine-head nature.
Obviously, if you can, try both on (one on each wrist), and see which one "speaks to you" louder.
-Dean